This invention relates to woven papermakers' fabrics and especially to forming fabrics, including those known as fourdrinier wires.
In the conventional fourdrinier papermaking process, a water slurry or suspension of cellulosic fibers, known as the paper "stock" is fed onto the top of the upper run of a traveling endless belt of woven wire and/or synthetic material. The belt provides a papermaking surface and operates as a filter to separate the cellulosic fibers from the aqueous medium to form a wet paper web. In forming the paper web, the forming belt serves as a filter element to separate the aqueous medium from the cellulosic fibers by providing for the drainage of the aqueous medium through its mesh openings, also known as drainage holes, by vacuum means or the like located on the machine side of the fabric. After leaving the forming section, the paper web is transferred to the press section of the machine, where it is passed through a series of pressure nips formed by cooperating press rolls to remove still more of its moisture content and finally to the dryer section for further moisture removal.
Such papermakers' fabrics are manufactured in two basic ways to form an endless belt. First, they can be flat woven by a flat weaving process with their ends joined by any one of a number of well known methods to form the endless belt. Alternatively, they can be woven directly in the form of a continuous belt by means of an endless weaving process. In a flat woven papermakers' fabric, the warp yarns extend in the machine direction and the filling yarns extend in the cross machine direction. In a papermakers' fabric having been woven in an endless fashion, the warp yarns extend in the cross machine direction and the filling yarns extend in the machine direction. As used herein the terms "machine direction" and "cross machine direction" refer respectively to a direction equivalent to the direction of travel of the papermakers' fabric on the papermaking machine and a direction transverse to this direction of travel. Both methods are well known in the art and the term "endless belt" as used herein refers to belts made by either method.
Effective sheet support and lack of wire marking are important considerations in papermaking, especially for the forming section of the papermaking machine where the wet web is formed. The problem of wire making is particularly acute in the formation of fine paper grades where the smoothness of the sheet side surface of the forming fabric is critical as it affects paper properties such as sheet mark, porosity, see through, pin holing and the like. Accordingly, paper grades intended for use in carbonizing, cigarettes, electrical condensers, quality printing and like grades of fine paper have heretofore been formed on very fine woven forming fabrics or fine wire mesh forming fabrics. In order to ensure the good paper quality required, the side of the papermakers' fabric which contacts the paper stock should provide high support for the stock, preferably in the cross machine direction because paper fibers delivered from the headbox to the forming fabric are generally aligned in the machine direction more so than they are in the cross machine direction. Trapping these paper fibers on the top of the forming fabric during the drainage process is more effectively accomplished by providing a permeable structure with a co-planar or bicrimped surface which allows paper fibers to bridge the support grid of the fabric rather than align with the support grid.
Such forming fabrics, however, may often be delicate and lack stability in the machine and cross machine directions, leading to a short service life. Abrasive and adhesive wear caused by contact with the papermaking machine equipment is a real problem. The side of the papermakers' fabric which contacts the paper machine equipment must be tough and durable. These qualities, however, most often are not compatible with the good drainage and fiber supporting characteristics desired for the sheet side of a papermakers' fabric.
In order to meet both standards, two layers of fabric can be woven at once by utilizing threads of different size and/or count per inch and another thread to bind them together. This fabric is commonly called a double layer fabric. Alternatively, fabrics have been created using multiple warps so that the fabric would have the desirable papermaking qualities on the surface that faces the paper web and desirable wear resistance properties on the machine contacting surface For example, papermakers' fabrics may be produced from two separate fabrics, one having the qualities desired for the paper contacting side and the other with the qualities desired the machine contacting side and then the two fabrics are joined together by a third set of threads. This type fabric is commonly called a triple-layer fabric. Generally, these structures do not possess the high level of stretch resistance desired in a papermaking fabric. Furthermore, the yarn that binds the fabric together will often produce a sheet mark, often from the long machine direction floats. Accordingly, no known fabrics have achieved the qualities necessary to meet those competing standards to produce superior paper.